The present invention relates to a multi-carrier transmission method, data transmission apparatus using this method, and mobile station and base station apparatuses incorporating this data transmission apparatus.
In mobile communications, there is a strong demand for multi-path fading countermeasures and improvement of the transmission quality. Multi-path fading can be overcome by reducing the symbol rate. On the other hand, implementing high-speed data transmission requires multi-carrier transmission. The best way to narrow sub-carrier intervals in multi-carrier transmission is the OFDM system. Conventional data transmission apparatuses using the OFDM system prevent leakage of unnecessary signals to outside a band by inserting null symbols at both ends of the band or placing restrictions on the band.
FIG. 1 is a block diagram showing the configuration of a data transmission apparatus using the OFDM system. In the data transmission apparatus shown in said figure, transmission data 1 is mapped by mapping block 2. For example, in the case of QPSK, mapping is carried out on 4 types of phase, 2 bits at a time, and in the case of ASK, mapping is carried out on 2 types of amplitude, one amplitude bit at a time. The mapped signal is serial/parallel-converted in serial/parallel conversion block 4 and then subjected to inverse Fourier transformation (IFFT) together with null symbol 3 in inverse Fourier conversion block 5. Through this processing, the signal placed on the frequency axis is converted to a time waveform.
FIG. 2 is a drawing showing the spectrum of a single sub-carrier centered on frequency f0. FIG. 3 shows these spectra lined up on the frequency axis. In this example, signals are carried on five sub-carriers and no signal is transmitted on 4 bands on each of the right and left sides. This band area where no signal is transmitted is called xe2x80x9cguard frequency band,xe2x80x9d which is implemented by means of null symbol 3.
The inverse-Fourier-transformed signal by inverse Fourier transformation block 5 is modulated through parallel/serial conversion in parallel/serial conversion block 6 to a time-series signal and further quadrature-modulated in quadrature modulation block 7 to a radio frequency signal and transmitted from transmission antenna 8. Thus, providing the guard frequency band where no signal is transmitted by means of null symbol 3 prevents leakage of unnecessary signal components to outside the band.
However, the data transmission apparatus above requires a lot of null symbols, which results in inconvenience such that the frequency utilization efficiency is reduced when carrying out frequency division especially on the uplink and that it is more vulnerable to distortion by multi-paths, etc.
FIG. 4 is a block diagram showing the configuration of a data transmission apparatus reinforced by guard intervals against distortion by multi-paths. In the data transmission apparatus shown in said figure, transmission data 1 is mapped by mapping block 2 and the mapped signal is serial/parallel-converted in serial/parallel conversion block 4, then subjected to inverse Fourier transformation together with null symbol 3 in inverse Fourier transformation block 5, and the signal placed on the frequency axis is transformed to a time waveform. The inverse-Fourier-transformed signal is converted to a time-series signal through parallel/serial conversion by parallel/serial conversion block 6. Then, guard intervals are inserted into this signal in guard interval insertion block 9.
As shown in FIG. 5, the guard interval is the last part of a valid symbol period added to the start of the symbol. This prevents distortion even if a delay wave lasting shorter than the guard interval may exist, making the signal resistant to multi-paths.
The signal with the guard interval inserted is quadrature-modulated in quadrature modulation block 7, converted to a radio frequency signal and transmitted from transmission antenna 8. Thus, inserting the guard interval makes the signal resistant to multi-paths.
However, the data transmission apparatus described above has a defect of its transmission efficiency being reduced due to the guard interval inserted to make the signal resistant to multi-paths, requiring a lot of null symbols. This results in a reduction of the frequency utilization efficiency especially when carrying out frequency division on the uplink.
FIG. 6 is a block diagram showing the configuration of a data transmission apparatus with the frequency utilization efficiency improved by narrowing the guard frequency through band restrictions. In the data transmission apparatus shown in said figure, transmission data 1 is mapped by mapping block 2 and the mapped signal is serial/parallel-converted in serial/parallel conversion block 4, then subjected to IFFT in inverse Fourier transformation block 5. The signal subjected to IFFT is parallel/serial-converted in parallel/serial conversion block 6 to a time-series signal and guard intervals are inserted into it in guard interval insertion block 9. The signal with the guard interval inserted is subjected to band restrictions in band restriction block 10 with the guard frequency band narrowed, then quadrature-modulated in quadrature modulation block 7 to a radio frequency signal and transmitted from transmission antenna 8.
In this case, in order to absorb distortion due to the band restrictions by band restriction block 10, providing guard intervals with a length corresponding to the impulse response length of a band restriction filter can remove distortion due to the band restrictions. Thus, band restrictions through band restriction block 10 can narrow the guard frequency part and improve the frequency utilization efficiency without null symbol insertion, etc.
However, the data transmission apparatus described above has a defect of its time efficiency being reduced because it requires extra guard intervals corresponding to the length of impulse response of the filter used for band restrictions.
FIG. 7 is a block diagram showing the configuration of a data transmission apparatus that disperses load between a guard frequency and guard interval. In the data transmission apparatus shown in said figure, transmission data 1 is mapped by mapping block 2 and the mapped signal is serial/parallel-converted in serial/parallel conversion block 4, then subjected to inverse Fourier transformation together with null symbol 3 in inverse Fourier transformation block 5. The signal subjected to IFFT in inverse Fourier transformation block 5 is parallel/serial-converted in parallel/serial conversion block 6 to a time-series signal and guard intervals are inserted into it in guard interval insertion block 9. This signal is subjected to band restrictions in band restriction block 10 with its guard frequency reduced, then quadrature-modulated in quadrature modulation block 7 to a radio frequency signal and transmitted from transmission antenna 8. Thus, carrying out both insertion of null symbol 3 and band restrictions makes it possible to reduce the number of null symbols compared to the case where only null symbol 303 is inserted and reduce the length of guard intervals to absorb distortion due to filtering compared to the case where only band restrictions are applied.
However, although this apparatus disperses load between the guard frequency and guard interval, it has the disadvantage of both the frequency utilization efficiency and time efficiency being reduced.
Thus, the conventional data transmission apparatus has problems that require solutions such as reducing the frequency utilization efficiency because it requires a lot of null symbols, being vulnerable to distortion by multi-paths because it has no guard interval, reducing the transmission efficiency due to the guard interval, reducing the time efficiency due to extra guard intervals, etc.
It is an objective of the present invention to provide a data transmission apparatus capable of improving the frequency utilization efficiency at both ends of a band, resistant to multi-paths because of its ability to introduce guard intervals, and capable of securing both the frequency utilization efficiency and time utilization efficiency by introducing band restrictions.
This objective is achieved by a multi-carrier transmission method that sets the distance between sub-carriers so that they may be orthogonal to one another, assigns a plurality of sub-carriers to a specific band and assigns low-speed variable signals to the guard frequency parts set at both ends of the specific band.